A supercritical fluid is a fluid having a density close to a liquid as well as a viscosity and a diffusion coefficient close to a gas. Since the supercritical fluid possesses both of diffusivity of the gas and substance solubility of the liquid, it has various effects as a reaction solvent.
Heretofore, the supercritical fluid has been utilized for extractive separation of effective ingredients and extractive removal of unnecessary ingredients such as extraction of hop extracts and fragrant materials, and decaffeination from coffee and tobacco by the use of its dissolving power. For example, caffeine-free coffee has been industrially manufactured by the utilization of supercritical carbon dioxide since around last half of 1970s.
In recent years, the supercritical fluids have been utilized for removal of impurities from chemical materials, products and the like, and concentration thereof, e.g., removal of unreacted monomers from a polymer, and concentration and dehydration of an alcohol. Furthermore, they have been also utilized for removal of a binder from ceramics, washing and drying of semiconductors and mechanical parts, and so forth. For example, Japanese Patent Application Laid-open No. 7-149721 discloses a method for purifying a bismaleimide compound which is characterized by subjecting an ether imide-based bismaleimide compound containing impurities such as aromatic hydrocarbon solvents used during the production of the same to an extractive removal treatment of the impurities in which the compound is brought into contact with carbon dioxide in a supercritical state or in a state close to the supercritical state having a pressure of 60 atm or higher and a temperature of 20° C. or higher.
In addition to the above, the supercritical fluids have been utilized for the formation of fine particles, thin films and microfilaments by rapid expansion (RESS method), e.g., the production of whisker-like fine particles of silica or the like. Moreover, they have been also utilized for the formation of fine particles and thin films by a conversion technique into a poor solvent (GAS method), e.g., strength enhancement (surface coating) of silica aerogel. For example, Japanese Patent Application Laid-open No. 8-104830 discloses a production method of fine particles for a paint wherein a polymer polymerization reaction solution in a polymerization step for producing a polymeric solid material for the paint is dissolved in a supercritical phase by the use of carbon dioxide and a polar organic solvent, and then rapidly expanded.
Meanwhile, heretofore, polymers such as the fine particles for the paint have been produced by a solution polymerization method or the like using a large amount of an organic solvent, in consideration of the control of a polymerization reaction rate, the handling of the polymerized product and the like. However, in the solution polymerization method, the polymer is formed in a solution state containing the solvent in about half amount, and hence a solvent removing step is necessary where the solvent is removed from the resulting polymer solution after the polymerization and the polymer is then dried, which means that the process is troublesome. In addition, it is also troublesome to treat the organic solvent which vaporizes in the solvent removing step.
Recently, it has been attempted to produce the polymer by the use of a supercritical fluid, particularly supercritical carbon dioxide as the solvent. In the case of using supercritical carbon dioxide as the solvent, the removal of the solvent after polymerization and the drying are unnecessary, and therefore the process can be simplified and costs can be reduced. Moreover, since no organic solvent is used, a burden to environment is also slight. In addition, carbon dioxide can be easily recovered and re-used as compared with the organic solvent. Furthermore, in many cases, there is a difference in solubility in carbon dioxide between the polymer and monomers, so that amounts of the unreacted monomers contained in the polymer which is the product are reduced by using supercritical carbon dioxide as the solvent, to enable the production of the more highly pure polymer.
As a production method of a polymer by the use of the supercritical fluid, for example, Japanese PCT Patent Application Laid-open No. 7-505429 discloses a production method of a fluoro-polymer comprising a step of solubilizing a fluoro-monomer in a solvent comprising supercritical carbon dioxide, and a step of thermally polymerizing the fluoro-monomer in the solvent in the presence of a radical polymerization initiator to produce the fluoro-polymer.
Japanese Patent Application Laid-open No. 2000-26509 discloses a production method of a fluoro-polymer wherein at least one fluorinated monomer is thermally polymerized in supercritical carbon dioxide by the use of dimethyl (2,2′-azobisisobutyrate) as a initiator.
Japanese Patent Application Laid-open No. 2002-327003 discloses a production method of a fluorinated alkyl group-containing polymer which comprises the step of thermally polymerizing a radical polymerizable monomer component containing a fluorinated alkyl group-containing (meth)acrylate in an amount of 20% by weight or more by the use of supercritical carbon dioxide as a polymerization solvent.
Japanese Patent Application Laid-open No. 2001-151802 discloses a production method of polymer fine powder which comprises the step of carrying out thermal radical polymerization of a monomer composition containing an ethylenically unsaturated monomer having a carboxyl group such as (meth)acrylic acid in supercritical carbon dioxide to form the polymer fine powder.
Japanese Patent Application Laid-open No. 2002-179707 discloses a production method of polymer fine particles which comprises the step of carrying out thermal polymerization of a monomer such as methyl methacrylate in supercritical carbon dioxide by the action of a radical polymerization initiator which is a polymer having a specific structure substantially soluble in supercritical carbon dioxide.
In addition, Japanese Patent Application Laid-open No. 2002-128808 discloses a production method of a polymer which comprises the step of carrying out thermal radical polymerization of a polymerizable monomer such as methyl methacrylate or styrene in supercritical carbon dioxide in the presence of a specific non-polymerizable dispersant such as docosanoic acid or myristic acid.
Masanori Kobayashi et al., “Dispersion Polymerization of Vinyl Monomers Using Supercritical Carbon Dioxide”, “Sikizai (Coloring Material)”, 2002, Vol. 75, No. 8, p. 371–377 describes that poly(1,1,2,2-tetrahydroheptadecafluorodecyl acrylate) and poly(1,1,2,2-tetrahydroheptadecafluorodecyl methacrylate) obtained by a polymerization reaction using supercritical carbon dioxide as a solvent are used as surface active agents, and supercritical carbon dioxide is used as a solvent to carry out a dispersion polymerization of various acrylic monomers.
As mentioned above, there has been already investigated the production method of the polymer which comprises the step of carrying out thermal polymerization of a monomer in a supercritical fluid such as supercritical carbon dioxide, but a production method of a polymer which comprises the step of photo-polymerizing a monomer in the supercritical fluid is not known.
Meanwhile, in recent years, a polymer brush attracts attention owing to its unique morphology. The polymer brush has a structure where polymer chains, whose terminal is immobilized to a solid surface (by chemical bond or adsorption), are stretched in the direction perpendicular to the solid surface. The extent of the stretching of the polymer chains noticeably depends on a graft density.
The polymer brush is usually obtained by grafting polymer chains onto a solid surface by surface graft polymerization, especially surface-initiated living radical polymerization.
For example, Japanese Patent Application Laid-open No. 2001-131208 discloses a production method of a polymer brush base material which comprises a step of providing a base material to which one or more free-radical initiators each having a radical formation site at a distant position from the base material are covalently bonded, and a step of bringing the covalently-bonded base material into contact with a monomer under conditions for accelerating free-radical polymerization from the radical formation sites of the initiators to form a polymer brush.
Moreover, Japanese Patent Application Laid-open No. 2002-145971 describes a production method of a polymer brush by surface-initiated living radical polymerization. Specifically, the surface-initiated living radical polymerization comprises a step of immobilizing a polymerization initiator onto a solid surface by the Langmuir-Blodgett (LB) method or a chemical adsorption method, and then a step of growing a polymer chain (graft chain) on the solid surface by living radical polymerization (ATRP method). Japanese Patent Application Laid-open No. 2002-145971 describes that polymer chains having a regulated length and length distribution can be grown on the surface of a base material with an unprecedentedly high surface density by the surface-initiated living radical polymerization, and they are then swollen in a solvent owing to the high graft density to provide a film thickness comparable to a fully stretched chain length and to thereby realize a “polymer brush” state in a true sense for the first time. Furthermore, Japanese Patent Application Laid-open No. 2002-145971 describes that in the conventional surface-initiated radical polymerization, a radical once generated continues to grow until its irreversible termination to form a graft chain sequentially and hence graft polymerization in the vicinity of the graft chains previously grown is inhibited owing to the steric hindrance of the graft chains, but in the present system, polymerization proceeds in a living manner, that is, all the graft chains grow almost evenly and hence steric hindrance among neighboring graft chains is reduced, the fact being considered to be one cause of obtaining the high graft density.
In addition, Japanese Patent Application Laid-open No. 2002-145971 mentioned above discloses a nano-structural functional material wherein a chemical composition of a graft polymer chain constituting a graft polymer layer arranged on a base material by graft polymerization is converted into a multilayered structure in the film thickness direction by copolymerization with a different kind of a monomer or an oligomer, obtained by such surface-initiated living radical polymerization. Furthermore, Japanese Patent Application Laid-open No. 2002-145971 mentioned above also discloses a nano-structural functional material wherein a polymerization initiating portion (polymerization initiating group) of a molecule arranged on the surface of a base material is inactivated with a predetermined pattern in the film surface direction and then the polymerization initiating portion not inactivated is subjected to graft polymerization to arrange a graft polymer layer with a predetermined pattern.
Furthermore, high density (dense) polymer brushes obtained by surface-initiated living radical polymerization are described in detail in Takanobu Tsujii, “New Development of Polymer Brushes”, “Mirai Zairyou (Future Materials)”, Vol. 3, No. 2, p. 48–55.
In this connection, in these conventional polymer brushes, the polymer chains (graft chains) are possible to have a fully stretched structure only in a good solvent, and the polymer chains (graft chains) has a fallen structure or a folded structure in a dry state or in a poor solvent.